An aluminum or aluminum alloy-coated steel material is widely used in the fields of automobile mufflers and building materials as a coated steel material having excellent corrosion resistance and high-temperature oxidation resistance. However, there is a problem with such an aluminum or aluminum alloy-coated steel material as described above in that the steel material experiences very high coating elution rate to be easily corroded and cannot demonstrate satisfactory corrosion resistance in a wet environment, although generation of corrosion products is relatively mild and stable. Thus, the steel material exhibits good corrosion resistance in a dry environment.
In view of this, JP-A 2000-239820 discloses for the purpose of improving corrosion resistance of an aluminum or aluminum alloy-coated steel sheet a hot-dip aluminized steel sheet having: an intermetallic compound coating layer provided on a surface of a base steel sheet, containing Al, Fe, Si, and having ≦5 μm thickness; and a coating layer provided on a surface of the intermetallic compound coating layer and composed by weight % of 2 to 13% Si, >3% to 15% Mg and the balance which is substantially Al.
JP-B 4199404 discloses a steel sheet provided with a hot dip Al-based coating and having good corrosion resistance, characterized in that the hot dip Al-based coating is a hot dip Al—Mg—Si based coating layer formed on a surface of the base steel sheet and containing, by weight %, Mg: 3 to 10% and Si: 1 to 15%, and the balance Al with incidental impurities, wherein: the Al—Mg—Si based coating layer has a metallic structure composed of at least an Al phase and an Mg2Si phase; and the long diameter of the Mg2Si phase is controllably 10 μm or less.
Further, WO 2000/56945 discloses a surface treated steel material having aluminum-based coating layer, characterized in that: the Al-based coating layer formed on a surface of the base steel material contains massive intermetallic compound bodies composed of at least one type of Group IIa (alkaline-earth metal) element and at least one type of Group IVb element; and the long diameter of each intermetallic compound body is at least 1 μm and the ratio of the short diameter with respect to the long diameter is at least 0.4.
However, the coated steel materials of JP '820, JP 404 and WO '945 still have problems described below, respectively.
Specifically, the coated steel material of JP '820 has a problem in that massive Mg2Si phase or Al3Mg2 phase is precipitated in the coating layer and serves as the origin of dissolution, thereby facilitating localized dissolution of the coating layer. The coated steel material of JP '404 has a problem in that the hot dip Al—Mg—Si based coating layer experiences preferential dissolution of Mg2Si phase thereof. Thus, subsequent localized dissolution start from the vicinities of the preferential dissolution.
The coated steel material of WO '945 has a problem of preferential dissolution of the intermetallic compound phase and subsequent localized dissolution of the coating layer triggered by the preferential dissolution.
Also, we previously proposed a steel material having a sacrificial anticorrosive film, wherein the sacrificial anticorrosive film contains Al, Mg and Si, contents of Mg and Si are 6 to 10 mass % and 3 to 7 mass %, respectively, and a ratio of Mg/Si is 1.1 to 3.0, as disclosed in JP-A 2010-168645.
Corrosion resistance of a coated steel sheet improved to some extent as a result of development of the steel material of JP '645. However, corrosion resistance still locally deteriorates in the steel material of JP '645.
It could therefore be helpful to further improve corrosion resistance of a coated steel sheet by preventing localized deterioration of corrosion resistance in particular.